The present invention relates to a temperature-responsive, mobile shielding device between a getter pump and a turbo pump in an in-line arrangement, adapted for high vacuum systems.
It is known that the operation of the getter pumps is based on the chemical sorption of reactive gaseous species such as O.sub.2, H.sub.2, water and carbon oxides by means of systems made with non-evaporable getter materials (known in the art as NEG), generally in combination with other pumps for producing and maintaining high vacuum in an enclosed chamber. While the first step of high-pressure pumping is usually carried out by means of mechanical pumps (e.g. rotary pumps), high levels of vacuum can be obtained by means of getter pumps in combination with chemical-ion, cryogenic or turbo pumps. It is particularly advantageous the combination getter pump/turbo pump, showing a combination of different behaviours with respect to the atmospheric gases or anyhow gases to be eliminated; in particular, the getter pump used at room temperature has a very good sorption capacity for hydrogen which is the most difficult gas to be eliminated by the turbo pump. Such a combination is particularly useful when it is a matter of evacuating a working chamber used for high-vacuum operations, such as a particle accelerator or a chamber of a processing machine in the semiconductor industry.
It is also known that these advantages could reach the highest level by mounting the two pumps in series to each other, with the getter pump upstream of the turbo pump and co-axial therewith. However such an arrangement has resulted in some drawbacks, the most important of which derives from the fact that the non-evaporable getter material has to be activated at temperatures of about 500-600.degree. C., by means of radiation heating from the inside or by flowing of electric current in the getter elements; furthermore, in certain applications the getter material is maintained at temperatures of about 200-300.degree. C. (whereas, when the purpose is to have the highest sorption of hydrogen, as stated above, the getter material is caused to work at room temperature). The getter pump heating has the consequence of an indirect heating (mainly due to radiation) also of the turbo pump. This caused the blades of the latter to expand beyond the admissible tolerances (however negligible) for a good operation of the pump itself. In order to avoid this inconvenience, there was the possibility of increasing the distance between the two pumps, introducing stationary thermal shields between them or connecting said pumps to each other in a non co-axial manner, through an elbow-shaped element: however all these solutions resulted in an undesired reduction of the gas flow conductance, whereby the two pumps were generally mounted, by means of flanges, to two difference openings of the chamber to be evacuated, thus doing without the advantages consequent to arranging the two pumps directly in line and co-axially to each other.
With WO 98/58173 in the name of the same applicant, an attempt was made to overcome the inconveniences above by means of a getter pump being mounted upstream and in the proximity of a turbo pump, co-axial thereto and having such a structure to minimize the direct heating of turbo pump, at the same time reducing the possible loss of particles from the NEG pump, with a very small reduction of conductance. However the pump structure, formed of an elongated metal element as a zigzag-shaped wire, with porous non-evaporable getter material deposited by sintering thereon and having such a configuration to occupy a crown-shaped peripheral zone of a cylindrical cartridge being the support of the getter pump, has required a special getter pump to be expressly manufactured when it was expected its combined use with a turbo pump, thus being excluded the use of NEG pumps of normal production, which are less expensive and probably more efficient, but not designed for the specific use of working in combination with turbo pumps.